Data storage disk

ABSTRACT

Data storage disks containing optically readable information made from a copolymer having low water absorption and good optical and mechanical properties, said copolymer being at least ternary and comprising 
     (A) methyl methacrylate, and further comonomers selected from at least two of the groups 
     (B) ##STR1##  where R&#39; is hydrogen or methyl and R&#34; is hydrogen or alkyl, and (C) ##STR2##  where R&#39;&#34; is aliphatic or alicyclic hydrocarbon, and (D) styrene and/or alpha-methyl styrene.

This application is a divisional of pending application Ser. No. 894,714filed Aug. 8, 1986, now U.S. Pat. No. 4,709,000, which is in turn acontinuation of Ser. No. 562,710 filed Dec. 19, 1983 and now abandoned.

The present invention relates to acrylic resins exhibiting low waterabsorption. Acrylic resins may absorb water from a humid atmosphere orthrough other contact with water. The water absorption of acrylic glasssubstitutes depends on factors determined by the material (such assaturation, concentration and diffusion coefficient) as well as factorsdetermined by the environment (geometry of the article involved,concentration and pressure of the moist medium, storage time, andtemperature).

So far as the mechanical properties of acrylic resins are concerned,water has a certain plasticizing effect. Some less in tensile andflexural strength as well as in modulus of elasticity occurs, whereasimpact strength and bending angle are increased. However, in traditionalprocessing technology, the influence of water on the physical propertiesof acrylic resins has been regarded as posing no particular problems.For example, in Vieweg-Esser, Kunstoff-Handbuch ["Plastics Handbook"],Volume IX, "Polymethacrylate" (Hanser-Verlag, 1975), it is stated onpage 425:

"Since the water absorption of polymethyl methacrylate is relativelylow, especially at temperatures under 100° C., by comparison with manyother synthetic resins, such as polyamide or cellulose acetate, mostchanges in the physical properties of acrylic glass substitutes arewithin narrow limits which in actual use are often hardly perceptible."

In U.S. Pat. No. 2,477,293 from the year 1947, the polymers andcopolymers of aryl and substituted aryl acrylates and methacrylates aresaid to possess fluidity, elasticity, plasticity, tensile strength,electrical resistance, resistance to water, organic liquids and gases,transparency, etc. However, the patent offers no teaching of practicalvalue in this connection.

U.S. Pat. No. 2,129,685, issued in 1934, claims copolymers of phenylesters of methacrylic acid and polymerizable derivatives of acrylic ormethacrylic acid. Mention is further made of blends ofpolymethylmethacrylate (PMMA) and polymers or copolymers of phenylesters of acrylic or methacrylic acid.

Solid copolymers comprising 1 to 25 percent by weight ofalkoxyphenyl-methacrylates and -acrylates and monoethylenicallyunsaturated monomers such as methyl methacrylate, styrene, vinylacetate, etc. are part of the teachings of U.S. Pat. No. 3,409,589.

Thermoplastic resins comprising 40 to 99 percent of polycarbonate and 1to 60 percent of a vinyl copolymer formed from 60 to 98 percent ofaromatic vinyl monomers, 2 to 40 percent of aralkyl or naphthylacrylates or methacrylates, 0 to 3 percent of polyfunctional monomers,and 0 to 1 percent of allyl monomers are known from published Japanesepatent application 78 34 853 (see Chem. Abstr. 89, 130345q).

By and large, the prior art acrylic resins have met the requirementsimposed on them. Problems occasionally arise when water acts onrelatively thin material in a nonuniform manner, as for example, inaquariums whose walls are too thin. Nonuniform water uptake by thematerial, or the development of a moisture gradient, may result inwarping or buckling.

German patent publication No. 30 28 498 points to the problems which thedevelopment of a moisture gradient can create in the case of opticallyreadable information storage disks made from PMMA. In that publication,these problems are solved by minimizing water absorption in the formingoperation and in the subsequent processing steps.

The undesired effect of nonuniform water uptake by information storagedisks (the so-called "umbrella effect") is also dealt with in U.S. Pat.No. 4,310,919, which seeks to solve the problem by making these diskscompletely symmetrical.

The solutions proposed are plainly directed toward prevention of thedevelopment of a moisture gradient or toward internal compensation forits effects.

Thus there has been a need for a material whose optical and mechanicalproperties satisfy the stringent requirements which information storagedisks, and especially optically readable information storage disks, mustmeet and which does not give rise to the problems due to nonuniformwater absorption, or in which these problems are not a factor.

This need is met by the acrylic resins of the present invention. Theacrylic resins in accordance with the invention are polymers of at leastthree monomer groups (terpolymers), preferably with significantproportions of monomers containing aromatic groups. The proportions ofthe various monomer groups are chosen so that the structural order whichis observed with aromatic monomer units, and which is due to thetendency of the aromatic group to associate, is largely suppressed.

More in particular, the synthetic resins of the present inventioncomprise at least three comonomers, namely methyl methacrylate and atleast two further comonomers chosen from at least two different groupsof monomers designated below at (B)-(D) and, optionally, furthermonomers of types (E) and/or (F) and/or (G). The monomer types and theirpercentages by weight in the polymer or in the monomer mixture fromwhich the polymer is made are:

(A) 20-85 percent of methylmethacrylate;

(B) 0-50 percent of at least one monomer of the formula ##STR3## whereinR' is hydrogen or methyl, R" is hydrogen or alkyl having 1 to 7 carbonatoms, Ar is phenyl optionally substituted with alkyl having from 1 to 7carbon atoms, n is an integer from 1 to 6, and m is 0 or 1, but n and mare not both 1;

(C) 0-60 percent of at least one monomer of the formula ##STR4## whereinR'" is linear or branched hydrocarbon having 2 to 20 carbon atoms or isa cyclic non-aromatic (i.e. alicyclic) hydrocarbon having up to 12carbon atoms and at least 5 ring-carbon atoms;

(D) 0 to 40 percent of styrene and/or alpha-methyl styrene;

(E) 0 to 9.9 percent of 4-methylstyrene;

(F) further copolymerizable comonomers different from (A)-(E) whosesolubility in water at 25° C. does not exceed 50 g/liter, and

(G) still other copolymerizable monomers different from (A)-(F).

The acrylic resins of the present invention are distinguished by thefact that their water absorption under the conditions of DIN 50 015 doesnot exceed 1.0, and preferably 0.8, percent by weight.

Quite unexpectedly, the mechanical properties, and especially theoptical properties, of the polymer system to be used in accordance withthe invention satisfy even the exacting requirements which opticallyreadable information storage disks must meet. This is true especially oftheir birefringence, which usually sharply limits the use of monomerscontaining aromatic groups, including styrene itself. Since thebirefringence exhibited by test samples is largely determined byprocessing conditions, it should be noted that the copolymers of theinvention, which are composed of at least three monomer groups, may beformulated so that these polymers can be used to produce formed articlesby injection molding without the products having unduly highbirefringence (for example, a value greater than 100 nm). Such moldingcompositions can also be extruded.

In selecting the comonomers from the aforementioned individual monomergroups, care should be taken to set the proportion of the monomers fromgroups (B) to (E) high enough for the polymer produced therefrom to havethe desired low water absorption, for example, less than 1.0 percent.All monomers of groups (B) to (E) will contribute to a reduction in thewater absorption of the methyl methacrylate polymer. As a rule, themonomers of groups (D) and (E) containing aromatic groups have theeffect of sharply reducing the water absorption of the polymer. However,they should not be used in such an amount that the birefringence exceedsthe desired value, for example 50 nm. In order to obtain polymers withthe requisite low water absorption, further monomers, for example oftype (C), should be copolymerized.

It has been found that in general the copolymerization of aromaticmonomers such as styrene with two different comonomers, for example,comonomers of group (A) and of group (C), will yield polymers havingless tendency to form birefringent structures than those resulting fromcopolymerization with methyl methacrylate [monomer group (A)] alone.

Thus, the terpolymers of the invention are generally distinguished notonly by reduced water absorption but also by reduced birefringence incomparison with merely binary copolymers, for example of methylmethacrylate [group (A)] and styrene [group (D)] having an equal contentof aromatic groups.

Moreover, the use of monomers of group (B) is of advantage. While thesereduce the water absorption of the polymer, they lack the tendency,exhibited by styrene for example, to form birefringent structures.

However, the amount of these type (B) monomers should generally be lessthan 50, and more particularly less than 30, percent by weight of thetotal monomers. An upper limit is imposed on the amount of type (B)monomers mainly by the fact that these polymers will lower the glasstransition temperature of the polymers. For this reason, particularlypreferred members of monomer group (B) are the esters of methacrylicacid since these have a higher glass transition temperature than theesters of acrylic acid. 2-phenoxyethyl methacrylate, benzylmethacrylate, 2-phenylethyl methacrylate, 3-phenylpropyl methacrylate,and 1-phenylethyl methacrylate are especially suitable.

The use of larger amounts (for example, over 9.9 weight percent) of type(E) monomers (e.g. para-methylstyrene) is not part of the presentinvention since the use of these monomers is governed by special rulesand relationships.

Suitable type (C) monomers are primarily those which will reduce thewater absorption of the polymers without markedly lowering their glasstransition temperature. These are those esters wherein R'" is linear orbranched hydrocarbon group having from 2 to 20 carbon atoms, or analicyclic hydrocarbon having up to 12 carbon atoms and at least 5 ringcarbon atoms. Isopropyl methacrylate, cyclohexyl methacrylate, thevarious butyl methacrylates, and n-propyl methacrylate are particularlysuitable. As a rule, these monomers should not represent more than 60weight percent of the total polymer.

The type (F) monomers which may be included should be different fromtypes (A)-(E) and should have a solubility of less than 50 g of monomerper liter of water at 25° C. These monomers are selected from the groupconsisting of α-olefins such as butadiene, isoprene and isobutylene,halogen containing olefins such as vinylidene chloride, esters ofunsaturated dicarboxylic esters such as esters of itaconic, maleic andfumaric acids and particularly their methyl and ethyl esters andvinylesters of saturated carboxylic acids with three to eight carbonatoms in the acid portion. The type (F) monomers are added to improvemechanical properties of the resulting polymer in particularflexibility.

These monomers may be used in an amount from 0 to 30, and preferablyfrom 0 to 10, percent by weight of the polymer.

In addition, small amounts (generally less than 10 weight percent) ofmonomers not named under (A)-(F), and which enhance polymer cohesion maybe optionally included as type (G) monomers. They are selected from thegroup consisting of acrylonitrile and methacrylonitrile and crosslinkingmonomers having in the molecule more than one group capable ofpolymerizing free radically, for example divinyl benzene, acrylic andmethacrylic esters of polyols such as butandiol dimethacrylate, andmoreover the vinyl-, allyl and crotylesters of acrylic and methacrylicacids. (See H. Rauch-Puntigam and T. Volker in "Acryl- undMethacrylverbindungen" ("Acrylic and Methacrylic Compounds"),Springer-Verlag, 1967, pp. 184-185).

The inclusion of comonomers adding to polymer cohesion is particularlyfavorable whenever the resulting polymer needs to be protected againstorganic solvents. The optional concurrent use of type (F) and type (G)monomers generally serves to improve the mechanical properties of thecopolymers.

As a rule, the water absorption of the copolymers is best determined onthe formed articles produced therefrom, for example, optical storagedisks. However, to obtain data on the water absorption of the desiredcopolymers easily and quickly, the water absorption can beadvantageously determined on a specimen having a high surface area, forexample a ground bulk polymer or a bead polymer. The specimen is thenfirst dried to constant weight (at 60° C. for 24 hours). The waterabsorption is determined simply as the gain in weight of the previouslydried specimen. The measurements are made under standardized climaticconditions, in conformity with DIN 50 015 (humid) at 23° C. and 83%relative humidity after 24 hours of exposure, as such period of timewill suffice to procure maximum gain in weight in any case. In practiceit has been observed that a small sized sample (e.g. polymer beads)after 6 hours exposure has already attained its maximum gain in weight.After 24-hour storage under these standardized climatic conditions, thewater absorption can additionally be determined by Karl Fischertitration. Weight gain and Karl Fischer values generally are in goodagreement. (See also Table I.)

Birefringence is determined on a small injection molded plate about 1 mmthick, for example. The values given in Table I should be regarded asguide values since birefringence is known to depend in large measure onthe processing technique.

However, it is apparent than changing from a binary copolymer to aterpolymer results in lower birefringence. For example, a copolymer of20 parts of styrene and 80 parts of methyl methacrylate exhibits abirefringence of 126 nm, whereas the birefringence of a small injectionmolded plate produced under the same conditions and composed of 20 partsof styrene, 20 parts of cyclohexyl methacrylate, and 60 parts of methylmethacrylate is only 70 nm. The use of more than two different monomerunits thus makes it possible to produce copolymers less capable oforientation, thereby hindering the formation of birefringent structures.

The polymers of the invention are suitable for a number of uses,especially where low water absorption coupled with low birefringence andgood further optical properties as well as mechanical properties arerequired. As mentioned earlier, these requirements apply especially tostorage media for information that is to be read optically, and moreparticularly to optical storage disks. For example, the birefringence ofmaterial of a thickness of 1 mm, for example, from which optical storagedisks are to be made, should not exceed 100 nm. Particularly preferredare data-storage materials having a birefringence of less than 30 nm.

The polymers of the invention are well suited for the manufacture ofsuch storage media, and particularly of optical storage disks. They lendthemselves especially well to the manufacture of the optical digitaldisks used with the DOR (Digital Optical Recording) technique. Thestorage capacity of such disks usually is very high and may range from10⁹ to 10¹¹ bits, for example. Processing can be done conventionally,for example by casting or by the use of a molding composition. It isalso possible to produce such information storage disks by extrusion orespecially by injection molding. Moreover, these storage disks foroptically read information can also be produced directly bypolymerization of the monomer mixtures.

The monomer composition should be chosen so that the Vicat softeningtemperature (as determined in conformity with DIN 53 460) of thecopolymers is above 70° C., and preferably above 80° C.

The polymers of the present invention are produced by free radicalpolymerization by conventional methods.

In principle, the polymerization of the comonomers of the invention fromat least three classes of monomers can be carried out in bulk, insuspension, in emulsion, or in solution.

In suspension polymerization, the polymer solids can be isolated simplyby filtering off the beads, followed by washing and drying. Recovery inemulsion polymerization is by precipitation, freeze coagulation, orspray drying, for example. In solution polymerization, the polymersolids can be separated by precipitation, for example in a nonsolventsuch as methanol.

As a rule, polymerization is initiated by means of free radicalinitiators such as organic peroxides and peresters, such as dibenzoylperoxide or tert-butyl-per-2-ethyl-hexaneoate, or with azo compoundssuch as azoisobutyronitrile, or with still other initiators. Thepolymerization temperature is based on the rate of decomposition of theinitiator employed. For regulation of the molecular weight, suchpolymerization regulators as tert-dodecylmercaptan, 2-ethylhexylthioglycolate, etc., are generally used. Quite generally, thepolymerization methods known to be used for methyl methacrylate and itscopolymerization with other monomers can be employed. In thisconnection, reference is made also to Houben-Weyl, Methoden derOrganischen Chemie ("Methods of Organic Chemistry"), 4th ed., vol 14/1.

For the production of molded articles to be used as storage media foroptically readable information, the monomer mixtures of the invention,comprising monomers from at least three different groups, can bepolymerized directly with the aid of an initiator and optionally of aredox component. It may be possible to start out from a monomer/polymersystem. As a rule, however, the formed articles will be produced byinjection molding, or else by extrusion, followed by the release of theformed article. In addition, formed articles can be produced bypressing. Thus the molding composition can be processed by varioustechniques. While the polymer should be readily processable, which callsfor a low molecular weight, the degree of polymerization should not bebelow a certain minimum since otherwise the material would be toobrittle. As a rule, molecular weights greater than 50,000, are moreparticularly greater than 70,000 are therefore required. However,especially when the compositions are used in extrusion or in injectionmolding, excessively high molecular weights (>10⁶) should be guardedagainst as otherwise increased birefringence might be encountered. Aminimum of light transmittance is essential particularly when thematerial is to be used to store optically readable information. Thatminimum depends, of course, on the type of light used to read theinformation. It follows that the basic material should have extremelylow intrinsic absorption (self-absorption). As a general rule, the lighttransmittance of the material, as determined on a plate 3 mm thick,should be greater than 80 percent, and preferably greater than 90percent. With regard to the measurement of light transmittance,reference is made to DIN 5036.

A better understanding of the present invention and of its manyadvantages will be had by referring to the following specific examples,given by way of illustration.

EXAMPLE 1

Polymerization in bead form

The data given in Houben-Weyl, Methoden der Organischen Chemie, 4thedition, volume 14/1, are applicable to the bead polymerization ofmonomer mixtures containing methyl methacrylate.

In addition to high molecular weight dispersing agents such as polyvinylalcohol or sodium salts of styrene/maleic acid copolymers, powdereddispersing agents, for example aluminium hydroxide produced in situ, maybe used. (c.f. Houben-Weyl, Volume 14/1, loc.cit., pp. 406-429).

In a batch of 1,000 ml of distilled water containing one of the abovedispersing agents, a mixture of monomer, regulator, and initiatorconsisting of

300 g of methyl methacrylate,

100 g of 2-phenoxyethyl methacrylate,

50 g of cyclohexyl methacrylate,

50 g of styrene,

4 g 70% benzoyl peroxide, and

3 g of 2-ethylhexyl thioglycolate

is agitated for 3 hours at 80° C. to 85° C. Agitation then is continuedfor 2 hours at 90° C.

On completion of the polymerization, the batch is cooled and the beadpolymer is washed with distilled water and dried. Free flowing, clearpolymer beads are so obtained.

The water absorption of the polymer is advantageously determineddirectly on the beads.

For the production of injection molded plates, the beads are firstgranulated to a uniform particle size. A granulate of uniform particlesize is also used in the production of extrudates. For acharacterization of the polymer, see Table I below.

Vicat softening temperature: 85° C.

EXAMPLE 2

the procedure of Example 1 is used, but with a different monomercomposition:

300 g of methyl methacrylate,

100 g of cyclohexyl methacrylate,

100 g of 3-phenylpropyl methacrylate,

4 g of 70% benzoyl peroxide, and

3 g of 2-ethylhexyl thioglycolate.

With regard to water absorption and birefringence of the injectionmolded plates, see Table I below. Vicat softening temperature: 84° C.,light transmission (500-700 nm) : >90% (Measurements being performedusing sample of 3 mm thickness).

EXAMPLE 3

The procedure of Examples 1 and 2 is used but with a differenct monomermixture:

375 g of methyl methacrylate,

100 g of 3-phenylpropyl methacrylate,

25 g of isobutyl methacrylate,

4 g of 70% benzoyl peroxide, and

3 g of 2-ethylhexyl thioglycolate.

Viscat softening temperature: 81° C., light transmission (500-700 mn) :>90%

EXAMPLE 4

The procedure of Examples 1 to 3 is used, except that the monomercomposition is as specified below and the reaction time is 7 hours.

300 g of methyl methacrylat,

100 g of styrene,

100 g of cyclohexyl methacrylate,

5 g of 70% benzoyl peroxide, and

3g of 2-ethyl thioglycolate.

Vicat softening point: 100° C., light transmission (500-700 nm) : >85%.

EXAMPLE 5 (Comparative example)

The procedure of Example 4 is used, except that a merely binary monomermixture is chosen:

400 g of methyl methacrylate,

100 g of styrene,

5 g of 70% benzoyl peroxide, and

3 g of 2-ethyl thioglycolate.

Free flowing, clear bead polymers are so obtained. The birefringence ofthe injection molded plates produced therefrom is significantly higherthan the values exhibited by the polymers of Example 1 to 4. (Cf. TableI.)

Vicat softening point: 96° C.

EXAMPLE 6

Bulk polymerization

Polymerization of the monomer mixture of Example 2 is carried out inbulk without the use of an aqueous dispersant system. The initiator usedis a blend of lauroyl peroxide and benzoyl peroxide. Polymerization iscarried out in the 60° C. to 90° C. temperature range.

The water absorption and birefringence of this bulk polymer correspondto the values obtained with the bead polyer of Example 2.

                                      TABLE I                                     __________________________________________________________________________    Characterization of the polymers of Examples 1 to 5                           Polymer composition         Water absorption.sup.(1)                          (wt. %)                     (%)       Birefringence.sup.(4)                   Example                                                                            MMA ST                                                                              CMA TBMA                                                                              POEMA                                                                              PPMA                                                                              Grav..sup.(2)                                                                      K.F..sup.(3)                                                                       (nm)                                    __________________________________________________________________________    1    60  10                                                                              10  --  20   --  0.77 0.74 19                                      2    60  --                                                                              20  --  --   20  0.62 0.76  3                                      3    75  --                                                                              --  5   --   20  0.95 0.93  2                                      4    60  20                                                                              20  --  --   --  0.78 0.80 70                                      .sup. 5.sup.(5)                                                                    80  20                                                                              --  --  --   --  1.03 1.04 126                                     __________________________________________________________________________     Key:                                                                          MMA = Methyl methacrylate                                                     ST = Styrene                                                                  CMA = Cyclohexyl methacrylate                                                 IBMA = Isobutyl methacrylate                                                  POEMA = Phenoxyethyl methacrylate                                             PPMA = 3Phenylpropyl methacrylate                                             .sup.(1) Climatic conditions in conformity with DIN 50 015 (humid):           23° C., 83% relative humidity.                                         .sup.(2) Determined gravimetrially.                                           .sup.(3) Determined by the Karl Fischer method.                               .sup.(4) Determined on an injection molded plate 1 mm thick.                  .sup.(5) Comparative recipe comprising only two comonomers.              

EXAMPLE 7

Bulk polymerization leading directly to a carrier for optically readableinformation.

A mixture of monomers of the following composition

39.8 percent styrene

39 percent cyclohexyl methacrylate

21 percent methyl methacrylate

0.2 percent butandiol dimethacrylate

is polymerized according to Example 6 using lauroyl peroxide and benzoylperoxide as initiators directly to form a sheet of 1,5 mm thickness.

Vicat softening point: 97° C.

Birefringence (difference in optical paths) : >3 nm

Water absorption (climatic conditions in conformity with DIN 50 015(humid) : 23° C., 83% relative humidity: 0.3 percent.

What is claimed is:
 1. A storage disk for information that is to be readoptically comprising a transparent data storage medium which is an atleast ternary acrylic copolymer exhibiting low water absorption andconsisting essentially of(A) 20 to 85 percent of methyl methacrylate;(B) at least 10 percent of a monomer of the formula ##STR5## wherein R'is hydrogen or methyl,R" is hydrogen or alkyl having from 1 to 7 carbonatoms, Ar is phenyl or phenyl substituted with alkyl having from 1 to 7carbon atoms, n is an integer from 1 to 6, and m is 0 or 1, but n and mare not both 1; (C) at least 10 percent of a monomer of the formula##STR6## wherein R'" is linear or branched hydrocarbon having from 2 to20 carbon atoms, or alicyclic hydrocarbon having up to 12 carbon atomsand at least 5 ring carbon atoms; (D) 0 to 40 percent or styrene oralphamethyl styrene; (E) 0 to 9.9 percent of 4-methylstyrene; (F) 0 to10 percent of a copolymerizable monomer, different from (A)-(E) whosesolubility in water is less than 50 g per liter at 25° C. and which isan alpha-olefin, a halogenated olefin, an ester of an unsaturateddicarboxylic acid, or a vinyl ester of a C₃ -C₈ carboxylic acid; and (G)0 to 10 percent of acrylonitrile, methylacrylonitrile, or a crosslinkingmonomer having in its molecule more than one group capable of freeradical polymerization, all said percentages being by weight of saidcopolymer.
 2. A storage disk as in claim 1 wherein monomer (B) isselected from the group consisting of 2-phenoxyethyl methacrylate,benzyl methacrylate, 2-phenylethyl methacrylate, and 3-phenylpropylmethacrylate.
 3. A storage disk as in claim 1 wherein monomer (C) iscyclohexyl methacrylate.
 4. A storage disk for information that is to beread optically comprising a transparent data storage medium which is anat least ternary acrylic copolymer exhibiting low water absorption andconsisting essentially of(A) 20 to 85 percent of methyl methacrylate;(B) 0 to 50 percent of a monomer of the formula ##STR7## wherein R' ishydrogen or methyl,R" is hydrogen or alkyl having from 1 to 7 carbonatoms, Ar is phenyl or phenyl substituted with alkyl having from 1 to 7carbon atoms, n is an integer from 1 to 6, and m is 0 or 1, but n and mare not both 1; (C) at least 10 percent of a monomer of the formula##STR8## wherein R'" is linear or branched hydrocarbon having from 2 to20 carbon atoms, or alicyclic hydrocarbon having up to 12 carbon atomsand at least 5 ring carbon atoms; (D) at least 5 percent of styrene oralphamethyl styrene; (E) 0 to 9.9 percent of 4-methylstyrene; (F) 0 to10 percent of a copolymerizable monomer, different from (A)-(E) whosesolubility in water is less than 50 g per liter at 25° C. and which isan alpha-olefin, a halogenated olefin, an ester of an unsaturateddicarboxylic acid, or a vinyl ester of a C₃ -C₈ carboxylic acid; and (G)0 to 10 percent of acrylonitrile, methacrylonitrile, or a crosslinkingmonomer having in its molecule more than one group capable of freeradical polymerization, all said percentages being by weight of saidcopolymer.
 5. A storage disk as in claim 4 wherein monomer (B) isselected from the group consisting of 2-phenoxyethyl methacrylate,benzyl methacrylate, 2-phenylethyl methacrylate, and 3-phenylpropylmethacrylate.
 6. A storage disk as in claim 4 wherein monomer (C) iscyclohexyl methacrylate.
 7. A storage disk for information that is to beread optically comprising a transparent data storage medium which is anat least ternary acrylic copolymer exhibiting low water absorption andconsisting essentially of(A) 20 to 85 percent of methyl methacrylate;(B) at least 5 percent of a monomer of the formula ##STR9## wherein R'is hydrogen or methyl,R" is hydrogen or alkyl having from 1 to 7 carbonatoms, Ar is phenyl or phenyl substituted with alkyl having from 1 to 7carbon atoms, n is an integer from 1 to 6, and m is 0 or 1, but n and mare not both 1; (C) at least 5 percent of a monomer of the formula##STR10## wherein R'" is linear or branched hydrocarbon having from 2 to20 carbon atoms, or alicyclic hydrocarbon having up to 12 carbon atomsand at least 5 ring carbon atoms; (D) at least 5 percent of styrene oralphamethyl styrene; (E) 0 to 9.9 percent of 4-methylstyrene; (F) 0 to10 percent of a copolymerizable monomer, different from (A)-(E) whosesolubility in water is less than 50 g per liter at 25° C. and which isan alphaolefin, a halogenated olefin, an ester of an unsaturateddicarboxylic acid, or a vinyl ester of a C₃ -C₈ carboxylic acid; and (G)0 to 10 percent of acrylonitrile, methacrylonitrile, or a crosslinkingmonomer having in its molecule more than one group capable of freeradical polymerization, all said percentages being by weight of saidcopolymer.
 8. A storage disk as in claim 7 wherein monomer (B) isselected from the group consisting of 2-phenoxyethyl methacrylate,benzyl methacrylate, 2-phenylethyl methacrylate, and 3-phenylpropylmethacrylate.
 9. A storage disk as in claim 7 wherein monomer (C) iscyclohexyl methacrylate.